1. Field of the Invention
The present invention relates to liquid crystal display using ferroelectric liquid crystal.
2. Description of the Related Art
Since liquid crystal display has features that it is thin and is low in power consumption and other features, the use has been expanding in various articles from large-sized displays to portable information terminals and the development has been actively made. Conventionally, for liquid crystal display, a TN system, an STN multiplex driving system, an active matrix driving system in which thin film transistor (hereinafter, it may be referred to as “TFT element”) are used in TN, and others have been developed and made practicable. However, nematic liquid crystal is used therein; therefore, the response speed of the liquid crystal material is as small as several milliseconds to several tens of milliseconds and it cannot be said that these sufficiently cope with display of moving images.
On the other hand, ferroelectric liquid crystal exhibits a very short response time in order of microseconds, thus ferroelectric liquid crystal is suitable for high-speed devices. As such ferroelectric liquid crystal, there is well known a bistable liquid crystal suggested by Clark and Lagerwall and which has two stable states when no voltage is applied thereto. However, the liquid crystal has a problem that the liquid crystal has memory property but graduation display cannot be attained since the switching thereof is limited to two states, namely, bright and dark states.
In recent years, attention has been paid to ferroelectric liquid crystal, in which the liquid crystal layer thereof is stable in a single state (hereinafter referred to as “monostable”) when no voltage is applied thereto, as a liquid crystal which makes it possible to attain graduation display by the matter that the director (the inclination of the molecule axis) of the liquid crystal is continuously changed by a change in applied voltage so as to analogue-modulate the light transmission thereof (NONAKA, T., LI, J., OGAWA, A., HORNUNG, B., SCHMIDT, W., WINGEN, R., and DUBAL, H., 1999, Liq. Cryst., 26, 1599).
In general, as a method for having the mono-stability of the ferroelectric liquid crystal, there are a polymer stabilizing method for adding a ultraviolet curable monomer in he liquid crystal material, injecting into a cell, and curing so as to stabilize the alignment of the liquid crystal, or a method for raising the temperature of the ferroelectric liquid crystal to a temperature higher than the phase transition point, and then cooling down gradually.
However, according to the polymer stabilizing method, problems are involved in that the process is complicated, the driving voltage is high, or the like. Moreover, according to the latter method not using the polymer stabilizing method, two different domains in the layer normal direction (hereinafter it may also be referred to as the “double domains”) can easily be formed so as to give rise to the grave problem of the display with the black and white inverted at the time of the drive. As to the double domains, although a method for having the mono domain by the electric field induced technique (, which uses DC voltage during cooling process) of gradually cooling while applying the voltage, is known (PATEL, J., and GOODBY, J. W., 1986, J. Appl. Phys., 59, 2355.), there is a problem of the alignment disturbance in the case the ferroelectric liquid crystal again has a temperature higher than the phase transition point, and thus the practical usability is low.
Moreover, as a technique for align the liquid crystal, there also are the rubbing process, the photo alignment method, or the like. According to the rubbing process, which provides the alignment ability by rubbing the polyimide-coated surface, it is difficult to have the evenness at the time of processing a large area, and moreover, there are the problems of generating the static electricity, the dusts, or the like. On the other hand, according to the photo alignment method, which provides the alignment ability by arranging the molecules in a specific direction by irradiating a compound having the photo alignment property with an ultraviolet ray, or the like, it is advantageous in that the quantitative alignment process can be controlled without generation of the static electricity or the dusts, however, a problem is involved in that the device cost is required due to the need of the exposing process.
On the other hand, such liquid crystal display comprises two alignment substrates having an electrode layer and an alignment layer on a substrate, so as to have the alignment layers face with each other, with the space between the alignment substrates filled with a liquid crystal. Furthermore, in general, a polarizing plate for changing an incident non polarized light beam to a linear polarized light beam is attached to the outside of the substrates.
Accordingly, since the liquid crystal display utilizes the birefringence effect, for the visualization, a polarizing plate is required. However, since the polarizing plate is attached to the outside of the substrate, scattering of the light is generated at the interface between the substrate and the polarizing plate so that a problem arises in that the light transmission can easily be lowered.
Moreover, recently, particularly for the usage in the portable terminal, or the like, miniaturization is demanded so that a thin shape and a light weight are required for the liquid crystal display. To meet this demand, in the case the thin shape and the lightweight are to be realized by changing the conventional glass substrate to a plastic substrate, since the polarization is disturbed by the birefringence of the plastic substrate, the substrate to be used is extremely limited and expensive. Moreover, although the display quality deterioration due to the birefringence of the substrate can be prevented by disposing a polarizing plate inside the substrate, since the conventional polarizing plate is produced in general by containing an iodine to a drawing alignment film, a problem is involved in that such a polarizing plate cannot be formed inside the substrate.